Serena H. Wong

Capacitive Micromachined Ultrasonic Transducers for Therapeutic Ultrasound Applications

Therapeutic ultrasound guided by MRI is a noninvasive treatment that potentially reduces mortality, lowers medical costs, and widens accessibility of treatments for patients. Recent developments in the design and fabrication of capacitive micromachined ultrasonic transducers (CMUTs) have made them competitive with piezoelectric transducers for use in therapeutic ultrasound applications. In this paper, we present the first designs and prototypes of an eight-element, concentric-ring, CMUT array to treat upper abdominal cancers. This array was simulated and designed to focus 30-50 mm into tissue, and ablate a 2- to 3-cm-diameter tumor within 1 h. Assuming a surface acoustic output pressure of 1 MPa peak-to-peak (8.5 W/cm2) at 2.5 MHz, we simulated an array that produced a focal intensity of 680 W/cm2 when focusing to 35 mm. CMUT cells were then designed to meet these frequency and surface acoustic intensity specifications. These cell designs were fabricated as 2.5 mm x 2.5 mm test transducers and used to verify our models. The test transducers were shown to operate at 2.5 MHz with an output pressure of 1.4 MPa peak-to-peak (16.3 W/cm2). With this CMUT cell design, we fabricated a full eight-element array. Due to yield issues, we only developed electronics to focus the four center elements of the array. The beam profile of the measured array deviated from the simulated one because of the crosstalk effects; the beamwidth matched within 10% and sidelobes increased by two times, which caused the measured gain to be 16.6 compared to 27.4.

Finite element modeling and experimental characterization of crosstalk in 1-D CMUT arrays

Crosstalk is the coupling of energy between the elements of an ultrasonic transducer array. This coupling degrades the performance of transducers in applications such as medical imaging and therapeutics. In this paper, we present an experimental demonstration of guided interface waves in capacitive micromachined ultrasonic transducers (CMUTs). We compare the experimental results to finite element calculations using a commercial package (LS-DYNA) for a 1-D CMUT array operating in the conventional and collapsed modes. An element in the middle of the array was excited with a unipolar voltage pulse, and the displacements were measured using a laser interferometer along the center line of the array elements immersed in soybean oil. We repeated the measurements for an identical CMUT array covered with a 4.5-mum polydimethyl-siloxane (PDMS) layer. The main crosstalk mechanism is the dispersive guided modes propagating in the fluid-solid interface. Although the transmitter element had a center frequency of 5.8 MHz with a 130% fractional bandwidth in the conventional operation, the dispersive guided mode was observed with the maximum amplitude at a frequency of 2.1 MHz, and had a cut-off frequency of 4 MHz. In the collapsed operation, the dispersive guided mode was observed with the maximum amplitude at a frequency of 4.0 MHz, and had a cut-off frequency of 10 MHz. Crosstalk level was lower in the collapsed operation (-39 dB) than in the conventional operation (-24.4 dB). The coverage of the PDMS did not significantly affect the crosstalk level, but reduced the phase velocity for both operation modes. Lamb wave modes, A0 and S0, were also observed with crosstalk levels of -40 dB and -65 dB, respectively. We observed excellent agreement between the finite element and the experimental results

Feasibility of MR-temperature mapping of ultrasonic heating from a CMUT

In the last decade, high intensity focused ultrasound (HIFU) has gained popularity as a minimally invasive and noninvasive therapeutic tool for treatment of cancers, arrhythmias, and other medical conditions. HIFU therapy is often guided by magnetic resonance imaging (MM), which provides anatomical images for therapeutic device placement, temperature maps for treatment guidance, and postoperative evaluation of the region of interest. While piezoelectric transducers are dominantly used for MR-guided HIFU, capacitive micromachined ultrasonic transducers (CMUTs) show competitive advantages, such as ease of fabrication, integration with electronics, improved efficiency, and reduction of self-heating. In this paper, we will show our first results of an unfocused CMUT transducer monitored by MR-temperature maps. This 2.51 mm by 2.32 mm, unfocused CMUT heated a HIFU phantom by 14degC in 2.5 min. This temperature rise was successfully monitored by MR thermometry in a 3.0 T General Electric scanner.

Evaluation of wafer bonded CMUTs with rectangular membranes featuring high fill factor

Increasing fill factor is one design approach used to increase average output displacement, output pressure, and sensitivity of capacitive micromachined ultrasonic transducers (CMUTs). For rectangular cells, the cell-to-cell spacing and the aspect ratio determine the fill factor. In this paper, we explore the effects of these parameters on performance, in particular the nonuniformity of collapse voltage between neighboring cells and presence of higher order modes in air or immersed operation. We used a white light interferometer to measure nonuniformity in deflection between neighboring cells. We found that reducing the cell-to-cell spacing could cause bending of the center support post, which amplifies nonuniformities in collapse voltage to 18.4% between neighboring cells. Using a 2-D finite element model (FEM), we found that for our designs, increasing the support post width to 1.67 times the membrane thickness alleviated the post bending problem. Using impedance and interferometer measurements to observe the effects of aspect ratio on higher order modes, we found that the (1,3) modal frequency approached the (1,1) modal frequency as the aspect ratio of the rectangles increased. In air operation, under continuous wave (CW) excitation at the center frequency, the rectangular cells behaved in the (1,1) mode. In immersion, because of dispersive guided modes, these cells operated in a higher order mode when excited with a CW signal at the center frequency. This contributed to a loss of output pressure; for this reason our rectangular design was unsuitable for CW operation in immersion.

Encapsulation of Capacitive Micromachined Ultrasonic Transducers Using Viscoelastic Polymer

The packaging of a medical imaging or therapeutic ultrasound transducer should provide protective insulation while maintaining high performance. For a capacitive micromachined ultrasonic transducer (CMUT), an ideal encapsulation coating would therefore require a limited and predictable change on the static operation point and the dynamic performance, while insulating the high dc and dc actuation voltages from the environment. To fulfill these requirements, viscoelastic materials, such as polydimethylsiloxane (PDMS), were investigated for an encapsulation material. In addition, PDMS, with a glass-transition temperature below room temperature, provides a low Youngs modulus that preserves the static behavior; at higher frequencies for ultrasonic operation, this material becomes stiffer and acoustically matches to water. In this paper, we demonstrate the modeling and implementation of the viscoelastic polymer as the encapsulation material. We introduce a finite element model (FEM) that addresses viscoelasticity. This enables us to correctly calculate both the static operation point and the dynamic behavior of the CMUT. CMUTs designed for medical imaging and therapeutic ultrasound were fabricated and encapsulated. Static and dynamic measurements were used to verify the FEM and show excellent agreement. This paper will help in the design process for optimizing the static and the dynamic behavior of viscoelastic-polymer-coated CMUTs.

P1B-10 Advantages of Capacitive Micromachined Ultrasonics Transducers (CMUTs) for High Intensity Focused Ultrasound (HIFU)

In the past ten years, high intensity focused ultrasound (HIFU) has become popular for minimally invasive and non-invasive therapies. Traditionally piezoelectric transducers have been used for HIFU applications, but capacitive micro- machined ultrasonic transducers (CMUTs) have been shown to have advantages, including ease of fabrication and efficient performance. In this paper, we show the fabrication and testing of CMUTs specifically designed for HIFU. We compare the operation of these designs with finite element models. In addition, we demonstrate that CMUTs can operate under high pressure and continuous wave (CW) conditions, with minimal self-heating, a problem that piezoelectric transducers often face. Finally, we demonstrate MR-temperature monitoring of the heating created by an unfocused HIFU CMUT.

Characterization of cross-coupling in capacitive micromachined ultrasonic transducers

This paper analyzes element-to-element and cell- to-cell cross-coupling in capacitive micromachined ultrasonic transducers (cMUTs) using an interferometer. In a 1-D linear cMUT array immersed in oil, a single element was excited, and membrane displacements were measured at different positions along the array with an interferometer. Electrical measurements of the received voltage on each array element were also performed simultaneously to verify the optical measurements. The array was then covered with a polydimethylsiloxane (PDMS) layer, and the cross-coupling measurements were repeated. The cross-coupling levels for conventional and collapsed operation of the cMUT were compared. Since the cMUTs were immersed in oil, the optical measurements were corrected for acousto-optic interaction, and the results were reviewed in time-spatial and frequency- spatial domains. The main cross-coupling mechanism was due to the dispersive guided modes supported by the membrane periodicity. In both modes of operation, cross-coupling dispersion curves predicted a gradual reduction in phase velocity at higher frequencies. At lower frequencies, this phase velocity tended to approach 1480 m/s asymptotically. Better cross-coupling suppression was observed in the collapsed (-34 dB) than the conventional operation (-23 dB). The element-to-element cross-coupling experiments showed that a 5-µm PDMS layer reduced the measured cross- coupling levels down to -39 dB in the collapsed operation. were corrected to eliminate the acousto-optic interaction due to the refractive index of the oil and the pressure created in the oil (4). The optical time domain measurements were analyzed in the wave number-frequency (k-w) domain for the multi-mode wave propagation (5). Conventional and collapsed operations of the cMUT were compared, and the influence of a 5-µm polydimethylsiloxane (PDMS) layer covering the cMUT was investigated. The main cross-coupling mechanism was due to the dispersive guided modes. Interface waves (Stoneley-Scholte) and surface waves (Rayleigh) were relatively weak in cross-coupling (3). The dispersive guided modes were determined for conventional and collapsed operations and corresponding k-w diagrams were analyzed.

Design of HIFU CMUT Arrays for Treatment of Liver and Renal Cancer

We present the development of a capacitive micromachined ultrasonic transducer (CMUT) array for noninvasive focused ultrasound ablation of lower abdominal cancers under MR‐guidance. While piezoelectric transducers have been traditionally used for HIFU, recent advances in CMUT design have made them highly competitive. Not only are CMUTs cost effective, they allow fabrication flexibility and advantages in efficiency and bandwidth. Current imaging CMUTs have shown capability of HIFU operation through high power and continuous wave operation. In this paper, we will present the development of CMUT membranes designed specifically for HIFU. These membranes are piston‐like membranes fabricated by placing a thick layer of silicon or gold at the center of the membrane. The width of the piston layer is usually 60–85% of the membrane width and allows the membrane mass and elasticity to be controlled independently. It also increases the average displacement and average output pressure of the membrane. We patterned the...

Capacitive Micromachined Ultrasonic Transducer Arrays for Integrated Diagnostic/Therapeutic Catheters

In recent years, medical procedures have become increasingly non‐invasive. These include endoscopic procedures and intracardiac interventions (e.g., pulmonary vein isolation for treatment of atrial fibrillation and plaque ablation for treatment of arteriosclerosis). However, current tools suffer from poor visualization and difficult coordination of multiple therapeutic and imaging devices. Dual‐mode (imaging and therapeutic) ultrasound arrays provide a solution to these challenges. A dual‐mode transducer can provide focused, noncontact ultrasound suitable for therapy and can be used to provide high quality real‐time images for navigation and monitoring of the procedure. In the last decade, capacitive micromachined ultrasonic transducers (CMUTs), have become an attractive option for ultrasonic imaging systems due to their fabrication flexibility, improved bandwidth, and integration with electronics. The CMUT’s potential in therapeutic applications has also been demonstrated by surface output pressures as h...

1I-2 Capacitive Micromachined Ultrasonic Transducers for High Intensity Focused Ablation of Upper Abdominal Tumors

We present the development of a capacitive micromachined ultrasonic transducer (CMUT) array for noninvasive focused ultrasound ablation of lower abdominal cancers under MR-guidance. While piezoelectric transducers have been traditionally used for high intensity focused ultrasound (HIFU), recent advances in capacitive micromachined ultrasonic transducers (CMUTs) have made them highly competitive with regard to costs, fabrication flexibility, and performance. Even current imaging CMUTs have shown capability of HIFU operation through high power and continuous wave operation. In this paper, we will show our experiments with current imaging CMUTs operated in HIFU mode. In addition, we will show the design and development of CMUT membranes and a transducer array specifically for HIFU ablation lower abdominal cancers